If you don't use the vanity area with a chair, it makes sense to continue the cabinets wall to wall. That gives you a great storage area, and with new cabinets you can use deep drawers for more useful storage. Having a shorter vanity just leaves wasted space on the ends. However, if that is truly the only location for hanging towels, you might want the vanity shorter on the right side so you have room for either hooks or a towel bar to be hung at a better height.